1. Field of the Invention
The present invention concerns a process for preparing tertiary alkyl ether products which are used, in particular, as a components of motor fuels. The products contain, for instance methyl t-butyl ether, ethyl t-butyl ether, t-amyl methyl or t-amyl ethyl ethers and possibly heavier tertiary alkyl ethers. According to the process, the isoolefins, in particular the C4-C7 isoolefins of the feedstock are reacted with a suitable alkanol for preparing the corresponding ethers. These ethers are removed together with the bottoms product of the distillation-reaction system and, if necessary, they are further processed in order to prepare a motor fuel component. Unreacted alkanol is removed with the overhead product of the distillation.
2. Description of Related Art
In order to improve the anti-knocking characteristics of motor fuels without using organolead compounds, and in order to reduce the concentration of detrimental components in the exhaust gases, tertiary alkyl ethers are added to the fuels. The oxygen-containing ether group of these compounds has been found to improve the combustion process in a favorable way as far as the aforementioned aspects are concerned. Suitable alkyl tert-alkyl ethers are methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), t-amyl methyl ether (TAME), t-amyl ethyl ether (TAEE) and t-hexyl methyl ether (THME), just to mention a few examples. These ethers are prepared by etherification of a monovalent aliphatic alcohol with an isoolefin. These olefins include, but are not limited to isobutene, 2-methyl-1-butene (2M1B), 2-methyl-2-butene (2M2B), 2-methyl-1-pentene (2M1P), 2-methyl-2-pentene (2M2P) and 2,3-dimethyl-1-pentene (23DMP). The reaction can be carried out in a fixed bed reactor, in a fluidized bed reactor, in a tubular reactor or in a catalytic distillation column.
In a fixed bed reactor, the feed components are reacted in the presence of a solid catalyst particles, said catalyst particles being contained in a layer which remains unmixed, because the liquid flow rates are so low that the catalyst particles do not separate from each other. They form a so-called fixed bed. On the other hand, in a fluidized bed reactor, the flow rate of the liquid phase is so high that the catalyst particles float separately in the fluidized bed of the reactor.
When the etherification is carried out in a catalytic i.e. reactive distillation process, the catalyst particles can form a fixed or fluidized bed in the column. The particular benefit which can be obtained by the catalytic distillation process is that the reaction and the separation of the products take place in the same vessel.
The etherification reaction is an exothermic equilibrium reaction, and the maximum conversion is determined by the thermodynamic equilibrium of the reaction system. Typically, by carrying out reaction and separation in one and the same reactive distillation column, it is possible to obtain an about 99% conversion in the case of MTBE, whereas only a 95% conversion is obtainable in a fixed bed reactor. The improvement in conversion for heavier ethers is even more significant. In case of TAME the conversion increases from 65% to 90%.
Ion exchange resins can be used as catalysts. Generally the resin used comprises a sulfonated polystyrene/divinyl benzene based cation exchange resin (sulfonated polystyrene cross-linked with divinyl benzene) having particle sizes in the range from 0.1 to 1 mm.
In case of MTBE (or ETBE) there are mainly two types of processes available. Both types have been in commercial use for more than 15 years. In the first commercial process for MTBE fixed bed reactors are used. The reaction section is followed by distillation in order to separate unreacted components from formed ether. One of the unreacted components is methanol, which is then separated by means of a water wash and a distillation. This recovered methanol is normally recycled back to the reactor feed. This kind of process is explained in more detail in the patent U.S. Pat. No. 4,198,530.
In order to improve the economics of the process, part of the catalyst was placed into the product distillation column. The principle is called reactive distillation and it lead to increased ether conversion, because the reaction and the separation of feeds and products is performed simultaneously.
The simultaneous removal of reaction product drives the process beyond the chemical equilibrium barrier. This process has been described in a number of patents. Placing the catalyst within a distillation column has, unfortunately, also drawbacks, which originate from feed impurities, which are poisons to the ion-exchange catalyst used. In some cases, depending on the feed origin, these impurities have to be removed before the etherification process. Otherwise the catalyst activity is gradually lost making the unit performance uneconomical due to lower conversion levels. If the catalyst within the distillation column needs to be replaced, it always means that the whole unit has to be shut down. Another drawback is that the catalyst used inside column is much more expensive the one used in fixed bed reactors.
Some processes try to avoid this costly catalyst placement by using a fixed bed reactor, which is coupled to the distillation column as a side reactor. One example is described in the U.S. Pat. No. 4,503,265. For some reason, however, there has been hardly any commercial success with this kind of processes in the MTBE production.
The use of reactive distillation does not, however, eliminate the need for separation and recycling of the alcohol used as a second feedstock. These operations increase the required investment costs and also burden the economics by creating additional operating costs. Also, because of the alcohol recycle, any feed impurities which travel along with alcohol build up within the unit generating even more stringent feed pretreatment requirements.
There are three alternative TAME preparation processes available. Two of these are older and use the same principles as described above for MTBE-production.
The third and newest process for preparation of TAME and also of heavier ethers is described in our international patent applications WO 93/19031 and WO 93/19032. It uses the side reactor principle in order to avoid the costly catalyst placement inside a distillation column. The main difference between to that process and the two others is, however, that it does not need alcohol separation and recycle. This is possible by unique utilization of alcohol-hydrocarbon azeotropes within the distillation column. The process can also use C5-C7 hydrocarbons as a feedstock. Said process is in commercial use.
So far almost all commercial etherification units produce only one ether as main product with the exception of above described third TAME process. Simultaneous production of ethers from C4-C7 hydrocarbons leads to some problems for example regarding catalyst placement. If a reactive distillation process is used the reactants build up in different sections of the distillation column requiring a wider placement of catalyst and making the internal flows and increasing the size of the columns. Furthermore, the simultaneous production of MTBE and THME is not possible with reactive distillation, since MTBE and the C6 hydrocarbons, which form THME, have boiling points that lie within the same range.
If prior art processes are used for simultaneous mixed ether production (MTBE/TAME/THME), alcohol separation and recycle is required. Even the third TAME process requires an alcohol processing section, since the C4-C7 hydrocarbon feedstock contains too much C4 hydrocarbons which do not react and thus make the distillate flow significantly larger than with a C5-C7 hydrocarbon feedstock. Since alcohol leaves the distillation column in the form of an azeotrope, the amount of alcohol in the distillate of mixed ether processes is unacceptable for downstream processes like alkylation, thus requiring the alcohol to be separated from the distillate.
Similar problems are encountered with all alkanols.
The present invention aims at eliminating the problems associated with the prior art by providing a completely novel process for producing tertiary alkyl ethers.
The invention is based on the concept of operating the product distillation of a catalytic distillation reactor system in such a way that most, and preferably practically all, of the alkanol which is removed with the distillate is bound to hydrocarbons, which are lighter than any of the reactive or unreactive C4-C7 hydrocarbons present in the feedstock.
It is known per se that, e.g., methanol forms an azeotrope with the components present in etherification mixtures of tertiary alkyl ethers. There are several applications known, wherein the unreacted methanol is removed from the top of the distillation column together with C4 hydrocarbons. Prior art processes of this kind are described in, e.g., the Published German Patent Application No. 2,705,538, the Published European Patent Application No. 78,422, U.S. Pat. No. 4,198,530 and the Swedish Applications Laid Open Nos. 448,452 and 459,175.
According to the present invention, the hydrocarbons lighter than any of the reactive or unreactive C4-C7 hydrocarbons possibly present in the feedstock are mainly removed with the overhead product (distillate) of the distillation. Therefore, the overhead product withdrawn from distillation substantially contains an azeotrope formed by the C3 hydrocarbons and the alkanol used. The amount of C3 hydrocarbons in the distillate corresponds at least approximately to the amount of C3 hydrocarbons present in the feed. In this way, an essential part of the unreacted alkanol is removed in the form of said azeotrope. The bottoms product of the distillation contains virtually all inert or unreacted C4-C7 hydrocarbons present in the feedstock along with formed ethers.
In particular, the process according to the present invention is characterized by the following steps:
feeding a feedstock containing hydrocarbons, in particular C3-7, hydrocarbons, to a catalytic distillation reactor system,
reacting the C4-7 isoolefines of the feedstock with an alkanol in the presence of a catalyst to form tertiary alkyl ethers,
removing the alkyl ethers from the distillation reactor and substantially all of the unreacted and inert hydrocarbons with the bottoms product of the distillation, and
withdrawing an overhead product, which mainly contains an azeotrope of C3 hydrocarbons and the alkanol.
The C3 amount of the withdrawn overhead product corresponds essentially to the amount of C3 hydrocarbons in the feedstock, whereby an essential part of the unreacted alkanol is removed in the form of said azeotrope.
The ether product preferably comprises MTBE or ETBE and the hydrocarbon feed contains primarily reactive C4 hydrocarbons.